DESIGN OF COMPRESSOR ENDWALL VELOCITY TRIANGLES

Near the endwalls of multi-stage compressor blade rows, there is a spanwise region of low momentum, high entropy fluid which develops due to the presence of annulus walls, leakage flows and corner separations. Off-design this region, known as the endwall flow region, often grows rapidly and in practice sets the compressor's operating range. By contrast, over the operating range of the compressor, the freestream region of the flow is not usually close to its diffusion limit and has little effect on overall range. In light of these two distinct flow regions within a bladerow, this paper considers how velocity triangles in the endwall region should be designed to give a more balanced spanwise failure across the blade span. In the first part of the paper, the sensitivity of the operating flow range of a single blade row to variations in realistic multi-stage inlet conditions and endwall geometry is investigated. It is shown that the operating range of the blade row is largely controlled by the size and structure of the endwall 'repeating stage' inlet boundary layer and not the detailed local geometry within the blade row. In the second part of the paper the traditional design process is 'flipped'. Instead of redesigning a blade's endwall geometry to cope with a particular inlet profile into the blade row, the endwall region is redesigned in the multi-stage environment to 'tailor' the inlet profile into downstream blade rows. This is shown to allow an extra degree of freedom not usually open to the designer. This extra degree of freedom is exploited to balance freestream and endwall operating range, resulting in a compressor having an increased operating range of similar to 20%. If this increased operating range is traded with reduced blade count, it is shown that a design efficiency improvement of Delta eta similar to 0.5% can be unlocked.